The boiling point of a given liquid varies directly with which factor?

• A. Pressure
• B. Inversely with temperature
• C. Volume
• D. Mass

Answer: (A)

Boiling point depends on ambient pressure. A liquid boils when its vapor pressure equals the pressure surrounding it. If you increase that surrounding pressure, the liquid must be heated to a higher temperature before its vapor pressure can match the higher external pressure, so the boiling point rises. Conversely, at lower external pressure, boiling occurs at a lower temperature (as seen at high altitudes or in pressure cookers, where water can boil at temperatures above or below standard values depending on the pressure).

Volume and mass don’t directly set the temperature at which a liquid boils, so they don’t determine the boiling point in the same way pressure does. The statement that boiling point varies directly with pressure captures the correct relationship.